High Intensity Short Arc Xenon Lamps are used in a variety of projection applications such as theater film projectors, high-end slide projectors and, Digital Video and Data Projection.
The Xenon lamp's output light intensity is generally defined by lm=K×I(op)X with K being a constant and 1.2<X<2.5, and both K and X are a function of lamp design parameters. Lamp output is defined by operating current for any given Xenon lamp design. Generally, in order for the output of a short arc xenon lamp to remain constant in an application the operating current needs to be kept constant.
A number of short arc power supply topologies have been developed to operate DC xenon arc lamps and DC xenon/mercury arc lamps. The “constant current mode ballast” (CCM) is popular as it is designed to keep the operating current constant over life of the lamp. This approach is straightforward but has some limitations. One limitation is that the thermal load on the lamp increases over life as the operating voltage of xenon short arc lamps increases due to electrode burn back. This requires the cooling for the lamp to be variable or designed for worst thermal load condition over the life of the lamp. Although the CCM solution can be acceptable for a number of applications, a different operating mode, as discussed below, has developed to provide a more constant thermal load over life of the lamp. Another risk with a CCM power supplies is that it can cause the overall power consumption to exceed the maximum allowable power consumption over life of the lamp.
The “constant power mode ballast” (CPM) addresses some of the limitations of the CCM ballast, by calculating power consumption of the lamp and adjusting the operating current over life in such a way the operating power remains constant. Generally, as the operating voltage of the lamp increases over life, the CPM power supply decreases the amount of current. This is a process playing over many thousand hours; many designers prefer the resulting decrease of output light intensity, to the thermal load increase in the CCM configuration.
In some of the prior CPM approaches the operating current and operating voltage of the power supply or ballast to the lamp are used to implement the CPM topology. By multiplying current and voltage the operating power of the lamp is determined. This signal is compared to a set level of desired operating power required for the application. The resulting error signal is utilized to drive the operating current of the lamp. As the operating voltage of the lamp changes over time due to electrode burn back, so will this topology decrease drive current to keep the Vop*Iop product constant, where the power corresponds to Vop*Iop.
The typical CPM ballast does an excellent job at keeping the thermal load constant at the cost of allowing for a variable operating current drive as a function of the operating voltage of the load.
As long as the operating voltage changes gradually, the output of the lamp will change gradually without any perceptual effects on the operation of the xenon lamp in aforementioned applications. However, a problem can arise where there are rapid and/or unstable fluctuations in the voltage across the lamp.
Many short arc xenon lamps are used in projectors or applications where observation by eye or camera is required. Keeping the light output in these applications constant is a major requirement and often the stability of the system is defined as a percentage level of output over a frequency range. The average human eye is sensitive to intensity variations in the 0-30 Hz frequency band and is capable of dissolving smaller than 1% intensity variations of full scale in the 9-11 Hz range at comfortable brightness levels. Sensitivity to changes is reduced at either end of this spectrum. Any perceivable intensity change that's noticeable is commonly referred to as “flicker”.
It is clear that slow light output changes as caused by electrode burn back will not be noticed as the frequency of the change approaches 0 Hz. The design of a short arc lamp, is well known, and includes generally includes two electrodes, an anode and a cathode, disposed in a gas region. Aspects of different elements of short arc lamps are described for example, in U.S. Pat. Nos. 5,721,465; 6,181,053; and 6,316,867 each of which is incorporated herein by reference. When a trigger voltage is applied across the electrodes the gas becomes conductive, and a dc voltage applied across the electrodes, results in current being conducted through the gas, and light is emitted by the lamp. Over usage of the lamp, the metal of the electrodes can erode, which is referred to as burn back. Electrode burn back is generally a gradual process, and as the electrodes burn back the impedance of the lamp will generally increase.
There are also other more rapidly varying processes which change the impedance and arc stability over the life of the lamp. The effects of these processes on system stability have not been accounted for in many prior systems. Examples of some processes that affect the operating voltage of the lamp and contribute to the changing impedance of the lamp, include:
a) Aging
b) Cathode work function changes
c) Thermal system effects
d) Micro-phonics
e) Arc Jump and Arc Wander
f) Any transient combination of above
g) Electrode morphology changing lamp operating processes.
h) Electrode evaporation due to excessive inrush current.
The above processes contribute to voltage changes in the lamp, and the CPM type system 100 of FIG. 1 will automatically adjust the operating current output by the power to the lamp, so as to keep operating power constant. This automatic adjustment of the current output by the power supply will also affect the output of the lamp and magnify the instability.
FIG. 1 shows an illustration of prior art CPM control circuitry 100 for a power supply driving a lamp. Inputs 102, 104, from J1 are signals which are detected from the operation of the lamp. The signal 102 corresponds to a voltage detected across the lamp. The signal 104 corresponds the current transmitted through the lamp. The chip U1 is an analog multiplier circuit. The current signal 102 and the voltage signal 104 are input to pins 112 and 116 of the analog multiplier circuit U1. The product of these two signals is then output on pin 122 to J2 where it can be used to make adjustments to the power supply outputting current to the lamp.
Also it should be noted that the changing operating current of the lamp further contributes to variations in the internal processes of the lamp, which leads to further changes in the operation of the lamp. If any of these processes affects the impedance of the lamp, system feedback with oscillation can result. The frequency of the oscillation is function of the timescale of the impedance changing process and the associated input/output system gain. After all, an oscillator can be no more than an amplifier with time function dependent gain feedback from output to input.
The resulting AC+DC operation of the lamp can result in rapid degradation of the electrodes in addition to making the output of the lamp unsuitable for some applications. This accelerated degradation can be a run-away process that accelerates the aging of the lamp.
An additional aspect of the operation of xenon short arc lamp is providing circuitry necessary to start the operation of the lamp. It is well documented that xenon short arc lamps to require a “boost circuit” to reliably start the lamp over life of the lamp. Critical boost circuit parameters are open load voltage, boost circuit energy and RC time or boost peak current time.
Since the impedance during the ignition phase of the lamp is initially very high as the gas in the lamp is not initially conductive, and the impedance does not immediately drop to a low value (<1 ohm for most xenon lamps), a high “open-load” voltage is used to speed up ionization in the lamp to establish a stable plasma. Indeed, a higher voltage will develop a higher current for given impedance at any time.
The boost phase “widens” the narrow discharge streamer of conductive gas between the anode and the cathode of the lamp generated in the ignition or trigger phase. By depositing more energy in the streamer, the boost voltage drives the impedance of the lamp low enough for the DC phase to take over
FIG. 2a illustrates an embodiment of a prior lamp system and power supply of the prior art. Initially, a trigger pulse supply 202 having a voltage in the range of 30-45 kV, for example, applies a large voltage pulse through a pulse transformer 216 to the lamp 218. This trigger pulse applies initial energy to the gas between the electrodes of the lamp. Subsequent to the trigger pulse, which lasts for in the range 1 μsec, the boost voltage is applied to the lamp 218 for a time period in the range of 0.3-2.5 msecs.
A general illustration of typical boost circuit of the prior art is shown in FIG. 2a; it includes a high voltage source 204 (100-450 volt DC) charging a capacitor 213 which is in series with a resistor 214. A switch configuration (not shown) can be used to discharge this capacitor 213 through the same or a different resistor into the lamp after ignition. In another configuration a PTC (positive temperature coefficient resistor can be used, which operates to initially have a low resistance value allowing for transmission of boost current, and then as the temperature of the PTC increases the resistance of the PTC increases, and the boost current is decreased. The diodes 210 and 212 can be deployed to keep boost circuit and DC circuit separated. These diodes can also be typically part of the rectification circuit of the DC generating circuit; they are shown in the circuit of FIG. 2a to illustrate that to ignite a short arc xenon lamp of the type used in projector type of applications multiple DC and AC sources are combined or OR'ed together for all the transitions to happen. The capacitor 213 and the resistor 214 in series in 2A are the typical boost capacitor and boost resistor.
FIG. 2b illustrates the operation of a prior boost circuit. From a time period 0 to 1 μs (220) the trigger voltage is applied to the lamp. This trigger voltage results in little current through the lamp due to the high impedance. Subsequent to the trigger pulse a boost voltage is applied to the lamp by the capacitor of the boost circuit for period of time 222. In one embodiment this time period can range for 0.3 to 2 ms and this boost voltage, particularly during the early part of the life of the lamp will cause the current flowing through the lamp to exceed the desired operating current, for a period of time 224. As the lamp ages the period of time during which the current exceeds the operating current will decrease because the impedance of the aging lamp will increase. Once the boost voltage has been applied for a sufficient amount of time, the gas will be conductive enough for the lower voltage of the DC source 206 to be sufficient to sustain the operation current through the lamp. The problem with many prior boost circuit systems is that they can cause the lamp to age more quickly than necessary by causing an excess amount of current to flow through the lamp during start up. The choice for an excessive amount of current is actually determined by the “one fits all ” design concept of old art approaches as depicted in FIG. 2a. 